Today's obesity levels are more than three times what they were in 1980, when only 6% of men and 8% of women were obese.

Most people who become obese put on weight gradually between the ages of 20 and 40, but there is some suggestion that the path is set in early childhood.

"Overweight children are more likely to become overweight adults," says Susan Jebb, professor of diet and population health at the University of Oxford.

The risk of becoming obese is thought to start at an early age and obesity in a parent increases the risk of childhood obesity by 10%. In 2011, around three in 10 boys and girls (aged two to 15) were either overweight or obese and around 16% were obese. There were 62 people aged under 18 who had weight loss surgery between 2011-2013 compared with just one child in 2000.

The North East had the highest obesity rates in England at 13.5% during 2011/12, while the South West reported the lowest levels at 10.4%. Tamworth in Staffordshire, where nearly 31% of people were obese, had the unenviable title of being dubbed Britain’s fattest town by the media.

"Obesity is closely linked to deprivation levels," says Dr Alison Tedstone, Director of Diet and Obesity at Public Health England. "The association is especially strong with children. Children in poor communities are far more likely to be obese."

Income, social deprivation and ethnicity have an important impact on the likelihood of becoming obese. For example, women and children in lower socioeconomic groups are more likely to be obese than those who are wealthier.

What caused the obesity crisis?

Rising obesity is not the result of a national collapse in willpower. Studies have shown the environment has a major influence on the decisions people make about their lifestyle. Known as "obesogenic environments", these are places, often urban, that encourage unhealthy eating and inactivity.

The car, TV, computers, desk jobs, high-calorie food, and clever food marketing have all contributed to encourage inactivity and overeating. "Obesity is a consequence of the abundance and convenience of modern life as well as the human body's propensity to store fat," says Professor Jebb.

Research has shown that we have a natural tendency to store fat – it's a survival mechanism that helped early humans survive famine and food shortages. "The situation in which food is readily available for most people has arrived in a blink of an eye in evolutionary terms," says Professor Jebb.

Adults spend about six hours a day engaged in sedentary pursuits (watching TV and other screen time, reading and other low-energy activities). On average, men and women spend 2.8 hours watching television per weekday and this rises to about three hours on weekends.

The average distance a person walked for transport purposes has fallen from 255 miles a year in 1976 to 192 miles in 2003, while car use increased by more than 10%. Although people are travelling further to get to work, one in five journeys of less than one mile are made by car.

"Your likelihood of being active is shaped by the environment you live in," says Professor Jebb. "For example, you're more likely to ride a bike if there are safe and convenient cycle lanes."

In 2012, only 67% of men and 55% of women aged 16 and over met the government's recommendations for physical activity of 150 minutes a week. Among children aged five to 15, more boys (21%) than girls (16%) met the recommendation to do an hour of activity every day.

Leisure time is increasingly spent indoors whereas the incentives for outdoor play have fallen due to safety concerns and a lack of access to green spaces and sports facilities.

Longer working hours and more desk-bound jobs over the past decades have resulted in limiting opportunities for other forms of activity during the working day.

Breastfeeding, healthy weaning practices and the mother's own diet have all been linked to reduced obesity later in life although why this is the case has yet to be fully explained.

Obesity's health consequences

Being overweight or obese increases the risk of a many serious illnesses, such as type 2 diabetes, high blood pressure, heart disease, stroke, as well as cancer.

"People are dying needlessly from avoidable diseases," wrote Prof Stephenson in the AoMRC’s Measuring Up report.

Compared with a healthy weight man, an obese man is:

five times more likely to develop type 2 diabetes

three times more likely to develop cancer of the colon

more than two and a half times more likely to develop high blood pressure – a major risk factor for stroke and heart disease

An obese woman, compared with a healthy weight woman, is:

almost 13 times more likely to develop type 2 diabetes

more than four times more likely to develop high blood pressure

more than three times more likely to have a heart attack

People from some ethnic groups, including south Asians, who are more likely to be overweight and obese, also have a higher risk of type 2 diabetes and other weight-related illnesses.

A BMI of 30 to 35 has been found to reduce life expectancy by an average of three years, while a BMI of over 40 reduced longevity by eight to 10 years, which is equivalent to a lifetime of smoking.

"As a society, we are gradually realising that obesity poses just as serious a health threat as smoking," says Dr Tedstone.

Obesity has been blamed for about 30,000 deaths a year in the UK, 9,000 of those taking place before retirement age.

Alongside disease, obesity can affect peoples' ability to get and hold down work, their self-esteem and their wellbeing and mental health.

What are we doing about it?

Reversing the obesity trend will require society as a whole to think differently. For government and businesses, it means creating an environment that encourages healthier eating and physical activity. For individuals and families, it means eating less and moving more.

It will require a major shift in thinking, not just by government, but by individuals, families, business and society in general.

Currently, no country in the world has a comprehensive, long-term strategy to deal with the challenges posed by obesity.

The only country to have successfully reversed its obesity problem was Cuba, although it was the unexpected consequence of an economic downturn in the early 1990s. This caused severe food and fuel shortages, which resulted in an average weight loss per citizen of 5.5kg over the course of the five-year economic crisis. During this time there was a significant drop in prevalence of, and deaths due to, cardiovascular diseases, type 2 diabetes and cancers.

The UK government has recognised that past efforts have not succeeded in turning the tide and that a new approach is required.

In 2011, it published Healthy Lives, Healthy People (PDF, 575kb), a policy document setting out its vision for how society as a whole can work together to turn the tide on obesity by 2020. Some of the government's measures to help people make healthier choices include:

giving people advice on healthier food choices and physical activity through the Change4Life programme

improving labelling on food and drink to help people make healthy choices

encouraging businesses on the high street to include calorie information on their menus so people can make healthy choices

giving people guidance on how much physical activity they should be doing

The government has also asked businesses to play their part in helping everyone – from staff to customers – make healthier choices through its Responsibility Deal pledges.

Food manufacturers, retailers and the hospitality sector have committed to cutting down on the amount of fat, sugar and salt in popular food products, encouraging people to eat more fruit and vegetables, reducing portion sizes and putting calorie information on menus.

Each of us is ultimately responsible for our own health. We should be free to make choices about diet and physical activity for ourselves and for our families.

"However, supermarkets and food manufacturers are already making choices for us by deciding how much fat and sugar to put in their products and what items to stock and promote," says Professor Jebb.

"Government can play a positive role in working with the private sector to help people make healthier choices to prevent weight gain."

Given all the external causes of the obesity epidemic, for the individual, it boils down to the simple message: to lose weight, you need to eat less and move more.

If you need to lose weight, why not start your weight loss journey with the NHS Choices 12-week weight loss plan, which is designed to help you lose 0.5kg to 1kg (1lb to 2lb) a week.